1. Field of the Invention
The present invention relates to a gas engine having a pre-combustion chamber in a cylinder head and using a gas fuel such as natural gas.
2. Description of the Prior Art
In conventional gas engines having a pre-combustion chamber in the cylinder head, the pre-combustion chamber is made fuel-rich for ignition and combustion to ensure combustion in lean regions in a main combustion chamber thereby stabilizing combustion. The gas engine, however, uses a natural gas as fuel, that is, the fuel is in a gas phase. When a gas is drawn into the engine during the intake stroke and then compressed, the air-fuel mixture is compressed to high pressures and its temperature rises to so high a level that a self-ignition phenomenon (knocking) may result. Normally, the natural gas must have a compression ratio of less than 12 to avoid self-ignition. As for the thermal efficiency, the smaller compression ratio means a lower thermal efficiency.
In the gas engine with a pre-combustion chamber, to further stabilize combustion of lean fuel, it is conventional practice to install a control valve in a communication port communicating the pre-combustion chamber with the main combustion chamber and to close the control valve during the intake stroke so that only air is drawn into the main combustion chamber, with a natural gas introduced into the pre-combustion chamber. In the gas engine with a pre-combustion chamber, the control valve is opened immediately before the top dead center to admit air instantly into the pre-combustion chamber by a pressure difference between the main combustion chamber and the pre-combustion chamber, causing the natural gas and air to quickly mix and ignite and then to flow out from the pre-combustion chamber through the communication port into the main combustion chamber. In this way the lean fuel combustion is completed in a short time.
A gas engine disclosed in Japanese Patent Laid-Open No. 310550/1995 introduces a gas fuel such as natural gas into the pre-combustion chamber, compresses only the air drawn into the main combustion chamber to an increased compression ratio, detects the internal pressure in the pre-combustion chamber by a sensor such as piezoelectric element, operates a fuel supply valve based on the internal pressure to supply an appropriate amount of fuel according to the load and revolutions, and opens the control valve in the communication port, while the air in the main combustion chamber is heated to high temperatures, to admit the highly pressurized air from the main combustion chamber into the pre-combustion chamber to quickly mix the gas fuel in the pre-combustion chamber with the highly pressurized air, thereby igniting and burning the air-fuel mixture in a short period of time. Because the fuel in the pre-combustion chamber is too rich, it is burned in a way that limits the production of NOx, and the flames as well as fuel are rapidly ejected from the pre-combustion chamber into the main combustion chamber where the air-fuel mixture spread as uniformly as possible is burned in the secondary combustion in a short duration, thus reducing the production of NOx and HC, enhancing heat efficiency, and preventing self-ignition of gas fuel or knocking.
Compared with a direct injection type diesel engine, the diesel engine with a pre-combustion chamber has the advantage of a reduced amount of NOx generated though its thermal efficiency is lower. The greatest reasons why the engine with a pre-combustion chamber has a lower thermal efficiency than the direct injection type engine are that because the flames, after the primary combustion in the pre-combustion chamber, are ejected through the communication port connecting the main combustion chamber and the pre-combustion chamber and burned in the secondary combustion, the combustion time becomes long; that the communication port communicating the pre-combustion chamber with the main combustion chamber produces a throttle loss; and that a large air flow in the pre-combustion chamber results in a large heat dissipation loss. In the gas engine with a pre-combustion chamber, because the air-fuel mixture generation energy and the ejection energy, essential for combustion, are produced by the throttling or diameter reduction of the pre-combustion chamber's communication port, the passage area of the communication port cannot be increased, resulting in a large pumping loss. Further, because the mixture is made by a turbulent air flow, the thermal conductivity in the pre-combustion chamber is large increasing a cooling water loss.
In the vortex flow chamber type engines, the communication port connecting the pre-combustion chamber to the main combustion chamber is small and thus produces a throttle loss, reducing the engine output. If the communication port connecting the pre-combustion chamber and the main combustion chamber is inclined in a direction tangential to the pre-combustion chamber wall surface, not only is the air flow in the pre-combustion chamber activated, but there is no attenuation in the ejection energy of flames into the main combustion chamber after ignition, enabling the flames to reach the outermost circumference of the main combustion chamber in a short time. This improves air utilization, allows clean combustion with few noxious emissions and improves the engine output. In the case of a pre-combustion chamber having inclined sub-communication holes, under the condition that does not take into account the influences of swirl of air drawn into the main combustion chamber, the speeds of air passing through the sub-communication holes when the air enters into the pre-combustion chamber and when the air is ejected out after ignition are equal. If the diameter of the communication port is throttled to increase the ejection energy, the air flow or swirl generated in the pre-combustion chamber also becomes stronger.
Because the communication port connecting the main combustion chamber and the pre-combustion chamber is provided at a cylinder center or at an outer circumferential position, the distance that the ejected flow must travel becomes longer, leaving the mixing of air and fuel in the main combustion chamber insufficient, which in turn will cause HC emissions and smoke. Further, there is a problem that when a swirl-formed in the cylinder when the air is drawn in through the intake port-flows into the pre-combustion chamber through the communication port, the energy of the swirl cannot be fully utilized in the pre-combustion chamber because the communication port is throttled and inclined.